<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <script>
        function sum(a=60,b=30){


            console.log(a+b);
        }

        sum();
        sum(5);
        sum(5,10);
        sum(5,10,15)

    </script>
</head>
<body>



</body>
</html>